The present invention relates generally to the measurement of the arrival time and velocity of shock waves and projectiles, and more particularly to the use of optical pins which include fluid-filled microballoons as a light source and an optical fiber as a link to a light detector.
An important diagnostic in hydrodynamic testing is the "pin," which detects the arrival of a shock-wave front or a high-velocity surface (i.e., a projectile) at a particular location. Used in an array, pins can be used to describe the time dependence of a hydrodynamic event in three spatial dimensions and in time. In the past, electrical pins have been widely used. An electrical pin is the insulated tip of an electrically charged cable. When a pressure pulse impinges upon the pin, the pin is caused to become electrically conducting, thereby causing a short in the cable which results in an electrical impulse to be transmitted from the pin to an electrical detector. This pulse records the time at which the pressure pulse reaches the location of the pin. A major disadvantage to electrical pins is their inherent sensitivity to electromagnetic noise, rendering measurements unreliable in harsh electrical environments. Moreover, the instrumentation required to record the electrical impulse is quite expensive.
Optical pins have been used in the past. Typically, bare optical fibers which produce an optical pulse in response to an impact by a shock front or projectile were employed. The optical pulse produced may be due to the luminosity of the shock front or due to pressure-induced luminosity. The latter effect often produces an optical pulse that is too dim for applications requiring subnanosecond time resolution and for applications where the only available streak camera is a rotating mirror design with inadequate sensitivity. Further, the duration of the optical pulse from the luminous fiber is too long for optimal recording by some instruments.
Flash gaps represent an alternative method to pin detection of shock-wave fronts producing an optical signal. Typically, a flash gap consists of a thin, gas-filled volume enclosed by a plastic envelope. The target gas is generally air, argon, or xenon. The rapid compression of the gas under the interaction with the impinging shock-wave front causes the gas to luminesce brightly. After the shock-wave traverses the gas and impinges upon the plastic envelope, the optical pulse is terminated. As a result the shock-wave produces a short pulse of light. Pulse duration is controlled by the thickness of the gas-filled volume; that is, the path through which the shock-wave front must traverse.
In "Nanosecond Hydrodynamic Diagnostics Using Fiber Optic Probes and a Streaking Camera," by L. L. Shaw, R. R. Donaldson, J. R. Murchie, and T. J. Ramos, Proceedings of the 12th International Congress on High Speed Photography (Photonics), Aug. 1-7, 1976, Toronto Canada, SPIE Vol. 97, pages 256-262, the authors describe an optical pin which includes a small closed space filled with xenon gas at a pressure of about 1 atm. However, the sensing end of the optical fibers taught by the authors is both sophisticated and complicated, making their pin apparatus expensive and difficult to place into service. Moreover, the gas fill pressure, which controls the sensitivity of the pin, cannot be varied. Indeed, the authors cannot measure the pressure of fill gas after the gas is loaded. Further, the thickness of the membrane which serves as a light shield cannot be substantially reduced in thickness. The use of a transparent microballoon as a pressure vessel to provide a flash gap which is detected by a photodetector via an optical fiber is not contemplated by this article.